Some felt Evan stabbing his hands in his classroom should have created changes, so Evan wouldn't have returned to jail for killing Tommy, but this is evidence of a pattern we see repeatedly in "The Butterfly Effect." When Evan alters events that only involve himself, or his mother and father, there are no consequences and the future timeline he returns to is the same. When Evan revisits the mailbox bomb the first time, he burns himself with his cigarette but goes back to an unchanged reality.
When Evan travels back to meet his father and asks how time travel works, his father strangles him just like when he was a kid, and Evan returns to an unchanged reality where he is a college student. When Evan travels back to his kitchen as a child to find a knife to cut up the stick of dynamite while visiting Kayleigh and Tommy in their basement, he immediately jumps back to the reality where he is an amputee.
However, when Evan goes back to the past to alter events to help Kayleigh, Lenny, or Tommy, the consequences are devastating. In one alternate timeline, Lenny winds up in a mental institution, while Kayleigh is an addict and sex worker. In one timeline, Tommy, Lenny, and Kayleigh are happy and healthy, but Evan is an amputee and his mother is dying. Tommy dies in two timelines. In another, Kayleigh dies and Evan ends up in an institution.
